The present invention relates generally to resistance welding and thermocompression bonding systems, and the like, and more particularly, to line voltage compensation for AC resistance welding controllers employed in AC resistance welding and thermocompression bonding systems in which output power remains constant.
Prior schemes for line voltage compensation employed in AC resistance welding controllers have included analog and digital methods. Resistance welding controllers manufactured by Hughes Aircraft Company and Unitek, for example, have utilized an analog line voltage compensation scheme wherein weld phase angle varies with line voltage. In addition, Hughes Aircraft Company has employed a digital line voltage compensation scheme in certain resistance welding systems where line voltage is measured periodically and a compensation factor is adjusted based on the line voltage.
However, these schemes do not have the ability to compensate for the frequency response (transfer function) of the load circuit comprising welding pulse transformer, cables, and weldments. Without the ability to compensate for the aforementioned, traditional compensation methods are subject to overcompensation. When line voltage increases, and compensation is applied, the energy delivered to weldments decreases, or vice-versa.
Careful measurements have indicated that conventional line voltage compensation methods, either analog or digital (based on computational analysis only) is capable of overcompensating for line voltage variations. That is, a line voltage change (230 volts to 240 volts, for example) which would theoretically cause an 8.8% power increase actually yields a reduction of 3% after compensation. It is believed that the cause of overcompensation is a somewhat complex response between the load resistance, the transformer inductance, and the harmonic of the phase controlled, voltage waveform.
Therefore, it is an objective of the present invention to provide for a line voltage compensation method for use in welders, such as resistance welding and thermocompression bonding systems, and the like, that eliminates the above-described overcompensation problem.